Effects of tree species composition on arthropod interactions and on soil microbiomes

Abstract

Natural disturbances are predicted to be more severe with future climatic extremes, especially in the boreal zone, where the low tree diversity may worsen the scale of damage. Boreal forests are widely managed for production of tree resources, often resulting in forests dominated by one tree species. Mixed forests are suggested to be less susceptible to biotic disturbance from insect herbivores and pathogens, through bottom-up and top-down effects on the organisms’ populations. However, supporting evidence of the positive relationships between plant diversity and top-down effects have been found in agricultural systems but its relation to tree diversity remains inconsistent and context dependent in forests. Thus, investigating tree species compositions in boreal forests as a measure to decrease damage from biotic disturbance is important. In this thesis, I investigate the effect of tree species composition on the insect herbivore the European pine sawfly (Neodiprion sertifer), tree-dwelling arthropod community and soil microbiome all related to Scots pine (Pinus sylvestris), the most common tree species in Sweden. By using field experiments measuring habitat characteristics and manipulating tree species composition, I explore how tree species diversity affects predation pressure, abundance and richness of the arthropod community, and the community composition of the soil microbiome compared to pure pine stands. I found that (i) predation pressure of sawfly larvae was not affected by tree diversity but by local habitat heterogeneity, (ii) predation patterns from birds and arthropods on prey with different traits differed depending on habitat characteristics (iii) the arthropod community composition was not affected by tree species composition, and lastly (iv) tree species effect in the soil microbiome depend on fungal functional group and tend to depend on spatial scale. I conclude that tree species composition may have limited effects on top-down control on insect herbivores and community composition of arthropod predators, while local habitat heterogeneity may be of more importance in this study system, and that relationships detected at a local scale may be obscured at stand level as for the soil microbiomes.